1. Field of the Invention
The invention relates to a process for the free radical polymerization of ethylenically unsaturated monomers in an aqueous medium and to a device for carrying out the process.
2. The Prior Art
Processes for the polymerization of ethylenically unsaturated monomers in an aqueous medium, such as, for example, emulsion, suspension or microsuspension polymerization, are described in detail in the literature and are known to experts, for example, from the following publications: Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], 4th Edition, Volume XIV, Stuttgart (1961); Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Volume E 20, Stuttgart (1987); EP-B 35785, EP-B 78043 and EP-B 93936.
Free radical polymerization of ethylenically unsaturated monomers in an aqueous medium is as a rule carried out in polymerization containers, for example autoclaves, with stirring. For this reaction, the mixing water, the monomer fraction, buffer salts, emulsifiers, protective colloids and, if appropriate, seed latices, together with the initiator or, in the case of redox initiators, together with the initiator components, are initially introduced, in their entirety or in part, into the autoclave, mixed thoroughly and, if appropriate, heated up to the polymerization temperature, and the monomers are polymerized completely, with stirring and removal of the heat of polymerization. In this procedure, the individual components of the polymerization batch, such as, for example, monomers, emulsifiers and/or initiators, can also be metered in subsequently. At the end of the polymerization, the unreacted monomers are removed, with utilization of the heat of polymerization or external introduction of energy, or the contents of the autoclave are drained into a downstream container, where the residual monomers are removed from the resulting polymer latex or the polymer suspension.
The wall deposits which form during the polymerization are a great problem in the free radical polymerization of ethylenically unsaturated monomers. They impede removal of heat from the reaction container and must be removed from the internal wall of the container by expensive cleaning processes after each batch.
High pressure water which is sprayed onto the container wall under, for example, 200 bar by rotating jets is generally used for cleaning the container wall. For this purpose, either the containers are opened and the corresponding cleaning equipment is inserted through a container opening, for example the open manhole, or cleaning equipment permanently installed on the container is used. Waste products and polluted waste water are obtained by this procedure, and must be removed; furthermore, these cleaning operations are time-consuming. Such a procedure is mentioned, for example, in GB-A 1557928.
To prevent wall deposits during free radical polymerization of ethylenically unsaturated monomers in the aqueous phase, a large number of wall coating compositions which are applied to the autoclave wall before the polymerization and are said to prevent the formation of wall deposits due to their inhibiting action have therefore been proposed. Coating the autoclave wall is described, for example, in U.S. Pat. No. 4,661,569. The disadvantages of such wall coating compositions are their cost, the time and apparatus needed for application, and their limited effectiveness during emulsion polymerization, since the emulsifier systems employed in this process accelerate detachment of the coating from the container wall.